Radiotelephones, which are well known in the art, generally refer to communications terminals which can provide a wireless communications link to one or more other communications terminals. Such radiotelephones are used in a variety of different applications, including cellular telephone, land-mobile (e.g., police and fire departments), and satellite communications systems.
Many radiotelephones, and in particular handheld radiotelephones, employ retractable antennas which may be extended out of, and retracted back into, the radiotelephone housing. Typically, such retractable antennas are electrically connected to a printed circuit board located within the housing of the radiotelephone that contains signal processing and other radio frequency circuitry. In order to maximize the transfer of power between the antenna and this radio frequency circuitry, the antenna and the radio frequency circuitry are typically interconnected such that the impedance of the antenna and the signal processing circuit are substantially matched. As many radiotelephones use 50 ohm impedance coaxial cable or microstrip transmission lines to connect the antenna to the radio frequency circuit, such matching typically comprises mechanically adjusting or electrically tuning the antenna so that it exhibits an impedance of approximately 50 ohms at its connection with the coaxial cable or microstrip transmission line.
Unfortunately, however, matching the impedance of a retractable antenna is more difficult, as the impedance exhibited by the antenna is generally dependent on the position of the antenna with respect to both the housing of the radiotelephone and the printed circuit board which contains the radio frequency circuitry. As these respective positions change when the antenna is moved between the extended and retracted positions, the antenna typically exhibits at least two different impedance states, both of which should be matched to the 50 ohm impedance of the feed from the printed circuit board. Accordingly, with retractable antennas, it is generally necessary to provide an impedance matching system that provides an acceptable impedance match between the antenna and the radio frequency circuitry both when the antenna is retracted and extended.
While it is generally desirable to optimally match the impedances of the antenna and the radio frequency circuitry, countervailing concerns are the physical size and expense associated with providing such matching. Most popular handheld telephones are undergoing miniaturization, such that many contemporary radiotelephones are as small as 11-12 centimeters in length, with correspondingly smaller printed circuit boards. Unfortunately, as the printed circuit board decreases in size, the amount of space which is available to support desired operational and performance parameters of the radiotelephone, including the space available for any impedance matching circuitry, is generally reduced. Thus, it is preferable that any matching circuit or components use, at most, minimal space on the radio frequency printed circuit board.
A number of different matching techniques are conventionally used with retractable antennas. For instance, many radiotelephones with retractable antennas employ dual impedance matching circuits, one of which is associated with the extended antenna position and the other with the retracted position. These matching systems typically comprise two or more resonant circuits and switches for switching between these circuits as a function of the position of the antenna. Other radiotelephones only provide a single matching circuit (which is switched in when the antenna is in the extended position), and operate without the benefit of any matching circuit when the antenna is in the retracted position. In other designs, a full half-wavelength (.lambda./2) antenna may be used so that the antenna radiates as a full half-wavelength structure in the extended position and as a quarter-wavelength (.lambda./4) antenna in the retracted position (as the retracted portion of the antenna does not radiate). With this arrangement, impedance matching is typically only required in the extended position, as the antenna may be designed to have a natural impedance reasonably close to 50 ohms in the retracted position. Still other radiotelephones use parasitic elements or printed transformer segments to match the impedance of the antenna to the radio frequency circuit board. However, each of the aforementioned techniques require some sort of matching means, which in turn requires space within the housing (or antenna) for matching components, and which additionally increases the overall cost of manufacturing the radiotelephone.
The aforementioned matching problems are further compounded in "dual-band" radiotelephones that are designed to transmit and receive signals in two or more widely separated frequency bands, such as the radiotelephones used with various satellite communications systems that employ widely separated transmit and receive frequency bands. As the impedance seen at the base of the antenna is usually a function of frequency, antenna systems for such dual-band radiotelephones typically require separate matching networks for each of the two frequency bands of operation. Accordingly, if retractable antennas are used on such radiotelephones, it is often necessary to provide as many as four matching networks to ensure that an acceptable impedance match is achieved at each frequency of operation and each possible antenna position (i.e., extended or retracted).
In addition to impedance matching problems, the performance of radiotelephones may also be negatively impacted by interactions between the antenna system and the housing of the radiotelephone, whereby energy is coupled from the antenna to the chassis of the phone where it is dissipated as heat. Similarly, the user of the telephone may also negatively impact the gain performance of the radiotelephone antenna if the user is sufficiently close to the antenna during operation of the radiotelephone. Accordingly, it may also be desirable to increase the isolation between the radiating structure and the chassis of the radiotelephone, to minimize the performance degradation resulting from these antenna/phone and antenna/user interactions.
In light of the above-mentioned problems with existing radiotelephone antenna systems, a need exists for radiotelephones with antenna systems that require minimal physical space within the housing of the radiotelephone for impedance matching purposes and which minimize the impact the chassis of the radiotelephone or the user may have on the performance of the antenna system.